Method for charging substantially horizontal bore-holes with explosives

Abstract: A method for charging explosives in substantially horizontal bore-holes, with a loading density reduced in relation to that corresponding to the complete fill up of the bore-hole diameter with the explosive in bulk form, comprising that a charging hose with an end opening is introduced into at least one bore-hole of a blasting round, that a pumpable and coherent bulk explosive is pumped through the charging hose at a controlled rate, that simultaneous with the pumping of explosive the hose is withdrawn at a controlled rate, that the pumping and withdrawal rates are adjusted to form a coherent string exiting from the hose end opening, said exiting string only partially filling up the bore-hole diameter. An apparatus for charging explosives in bore-holes in controlled volume amount per bore-hole length unit comprises, a vessel (31) containing a pumpable and coherent bulk explosive (32), a charging hose (45) adappted for insertion into the hole-hole, a conduit (38) connecting the vessel with the hose, pumping means (33,34) for moving the explosive from the vessel through the conduit and the hose at a controlled rate, hose moving means (44,48) allowing forward movement of the hose and withdrawal of the hose at a controlled rate and adjusting means (34,48) for setting the ratio between pumping rate and hose withdrawal rate.

Description: Technical field The present invention relates to a method for charging explosives in substantially horizontal bore-holes, with a loading density reduced in relation to that corresponding to the complete fill up of the bore-hole diameter with the explosive in bulk form. Background In many blasting applications it is desirable to have a charging method providing explosive of reduced and variable bulk strength. In driving tunnels or galleries careful blasting of the contour holes will give a substantially undamaged rock face with strongly reduced needs for subsequent repair and support work such as bolting, gunniting, concrete reinforcement etc. and the final profile will be true the design size. Similar considerations may arise in underground mining and stopig or for the purpose of limiting production of fines to meet certain after-processing constraints. Although numerous small and closely spaced bore-holes can be used to produce smooth fracture planes, the method is limited by practical and economical reasons and conventionally careful blasting has been carried out by partial charging of oversized boreholes with small-diameter cartridges or tubes. Another approach is the errangement of spatially separated and individually ignited deck charges at regular intervals in the borehole. The methods are expensive both in labour and equipment. Frequent problems are inconsistency in charging and uncontrolled coupling between explosive and rock. Detonation failures have also been experienced for certain explosives, supposedly due to precompression from forerunning shock waves in the free gas channel. Introduction of shells or spacers concentric with the charge have improved positioning but added to cost and complicated charging procedure. To meet the general trend towards wider boreholes and bulk charging of explosives also in connection with careful blasting, bulk explosives of strongly reduced energy concentration have been developed, such as ANFO mixed with porous lightweight material. The complete fill out of large drill holes with explosive places severe demands for energy reduction and the explosive often approaches its detonation limit. Although the positioning problems mentioned in connection with the packaged products are avoided with bulk explosives, the coupling to the rock surface is stronger and the blast result will be markedly dependent on any inhomogenity present in the explosive. These problems are pronounced by the pulverulent nature of the explosives used. The lightweight materials usually employed for energy reduction are not easily mixed with the heavier standard components of the explosive. Precautions taken at manufacture to secure thorough mixing are not sufficient since the components tend to seperate during transport and charging operation. The US patent 4 995 925 describes an improved composition of this kind in which the segregation problems are controlled per se. The general problems with bore-holes filled up with reduced explosives are not solved, however, nor is the need for use of a single explosive composition for multiple strength requirement met by such explosives. The US patent 5 105 743 on which disclosure the preamble of independent claim 1 is based, describes a method by which a standard blowable explosive is used to partially fill upp a bore-hole. The method is limited to granular and blowable explosives and is of limited use in for example wet environments or other situations when pumpable and coherent explosives are needed. The method requires different tools for different bore-hole diameters and tend to give uneven amounts along the hole. As prior art reference is also made to FR-A-2 584 179 which discloses a system for retarding the detonation of explosives while being transported. However, it does not disclose anything in connection with the charging of substantially horizontal bore-holes with fluid or viscous explosives. Unlike granular explosives, coherent and pumpable explosives of reduced diameter are susceptible to detonation propagation problems. Under proper detonation they tend to sustain a high detonation velocity, both unconfined and fully confined, which is not always consistent with cautious blasting requirements. Summary of invention A primary object of the present invention is to provide a method for charging and blasting substantially horizontal bore-holes with pumpable explosives in reduced amounts. Another object is to provide such a method suitable for cautious blasting. Still another object is to provide a method allowing charging of pumpable explosives in easily varied specific loadings for different bore-hole requirements in the blast. Yet another object is to provide such varied charging with essentially the same explosive. A further object is to reach the abovesaid objects highly independent of bore-hole sizes. A final object is to obtain the stated purposes with different kinds of pumpable explosives and under optimal utilization of their respective energy reduction capabilities. According to one aspect of the invention there is provided a method for charging explosives in substantially horizontal bore-holes, with a loading density reduced in relation to that corresponding to the complete fill up of the bore-hole diameter with the explosive in bulk form, which method is as defined in claim 1. The method of the invention can be performed by means of an apparatus for charging explosives in bore-holes in controlled volume amount per bore-hole length unit, which apparatus comprises a vessel containing a pumpable and coherent bulk explosive, a charging hose adapted for insertion into the bore-hole, a conduit connecting the vessel with the hose, pumping means for moving the explosive from the vessel through the conduit and the hose at a controlled rate, hose moving means allowing forward movement of the hose and withdrawal of the hose at a controlled rate and adjusting means for setting the ratio between pumping rate and hose withdrawal rate. By forming a string of pumpable explosive, only partially filling out a bore-hole diameter, several objectives are reached. The explosive itself need not be highly diluted, with corresponding problems, but energy reduction is accomplished by amount and string size. Variability in specific loading is obtained and specificly it is possible also to charge some bore-holes in their entiry with utilization of the full power of a bulk explosive. Yet, the most pronounced advantages are obtained in cautious blasting with thin strings of the explosive. It has been found that a pumpable bulk explosive string, uncoupled from the bore-hole wall and spacing devices, neither behaves as confined nor as unconfined, with high detonation velocities. Rather it detonates with a markedly reduced velocity and shock generation, perfectly meeting the requirements in cautious blasting. The charging method outlined and the detonation mechanism obtained sustains a stable and undisturbed detonation also in thin strings, contrary to previous experience. The method adapts to a great variety of pumpable bulk explosives, allowing selection of the proper explosive for each blasting environment, e.g. in respect of strength, water resistance, sensitivity etc. The method is compatible with both microsphere sensitized and gassed explosives. The latter explosive type may optionally benefit from the possibility of after-foaming into the free radial space without axial movements, thereby further Increasing the sensitivity or lowering the critical detonation string size. The method requires no auxiliary devices over the explosive itself. Further objects and advantages will be evident from the detailed description hereinbelow. Detailed description The basic feature of forming a cohesive bulk explosive string, only partially filling up the bore-hole diameter, can be used for any kind of horizontal or substantially horizontal bore-holes in which the string can be properly positioned and retained up to initiation of the blast. In this contact the term horizontal bore-holes or substantially horizontal bore-holes is to be understood to include also inclined holes insofar the string is stably retained therein. Although most explosives have a gap sensitivity sufficient to bridge and maintain reaction also over certain interruptions in the string, it is preferred that the string formed is substantially cohesive over the length considered without any larger thinnings or discontinuities. Smaller irregularities are of no significance and may to some extent be unavoidable due to roughness on the bore-hole walls and other disturbances. The principles of the invention may be used for charging the entire or only part of the bore-hole length. Generally it is preferred that the major part of bore-hole length is charged with a string according to the invention. The string may have a systematicly varying cross-section area over bore-hole length. A preferred kind of variation is to have a decreasing area from the bore-hole inner part towards hole opening in order to meet the requirement for higher amounts in the innermost part of the hole. In most applications though, it is preferred to have a substantially constant cross-sectional area. The method steps are adapted to give a string of above-said characteristics. The bore-hole is charged from the bottom or innermost part by pumping the explosive at a controller rate from a charging hose under simultaneous withdrawal of the hose at a controlled rate. By mutually adjusting the pumping and withdrawal rates the desired string amounts can be extruded from the hose end. Both rates can be varying over time to give either a varying or a constant exiting explosive amount although it is preferred to keep at least one of the rates constant. When extruding a string of varying cross-sectional area it is preferred to keep the withdrawal rate constant and when extruding a string of constant cross-section to keep both rates constant. Part of the bore-hole may be charged differently than with the string of the invention. Specificly igniting means in the form of detonators and/or primers are positioned in the bore-hole, commonly in the innermost part. In order to secure a safe ignition it is suitable to use an excess of explosive around the igniting means and preferably entirely fill up the bore-hole diameter around these devices. Similarly the outermost bore-hole parts may need less or no amounts of explosive. Excess charging can be obtained by a delay in hose withdrawal in relation to pump start and a reduction by slowing or stopping pumping. Partial charging is highly independent of absolute bore-hole diameter and the string charging of the invention may be utilized for broad size ranges. A non-limiting indication of suitable diameters is between 25 and 150 mm (1 and 6 inches) and preferably between 36 and 100 mm (1.5 and 4 inches). A viscous explosive may flow and adapt to bore-hole shape even if extruded as a circular string. Hence partial charging degree shall here be expressed as the exiting string cross-section area to bore-hole cross-section area. In broad terms the charging degree so stated may lie between 10 and 90 percent and preferably between 20 and 80 percent. The exact degree of partial charging depends on the purpose of the reduction. For the most preferred application in cautios blasting the lower charging degrees should be selected, such as between 10 and 75 percent or preferably between 15 and 60 percent. Too high degrees may give insufficient reduction and too low degrees insuficient breakage.In absolute terms string cross-section area may be between 1 and 20 sq.cm or preferably beween 2 and 15 sq.cm. As indicated, in partial string loading according to the invention it is possible, and in cautious blasting desirable, to strive for velocity of detonation (VOD) significantly lower than the velocity obtained both fully confined and fully unconfined. When utilizing this possibility the VOD may be between 25 and 75 percent, and preferably beween 30 and 60 percent of the VOD for the same explosive, in the same string size, detonated freely on the ground. It may be that the bore-hole string is to thin to be detonated freely and in that case the abovesaid values should be compared with the smallest string freely detonatable. In absolute terms the VOD may be between 500 and 3500 m/sec and preferably between 1000 and 2500 m/sec. Another application for the partial charging of the invention is to adapt charge strength to the specific need in each bore-hole, i.e. also drift holes and production holes, not particularily the contour holes. For this purpose a broader range of partial charging degrees can be used and in particular the higher charging degrees, such as 25 to 90 percent and preferably 30 to 75 percent. According to the invention at least one bore-hole is partially charged with a string for any of the above purposes. In order to utilize the flexibility of the invention it is prefered to charge several bore-holes with different charge ratios, in particular several bore-holes to be blasted in the same round. It is within the scope of the invention that any of such additional bore-hole is fully charged, i.e. to substantially 100 percent as above, in order to utilize the full breadth of the invention. It is within the scope of the invention that different explosives, e.g. with different strength, are used for different holes but the flexibility of the invention is best utilized if the same explosive is used for more than one hole and varying charge ratios. The explosive is a bulk explosive in order to avoid handling of cartridges or packages. Generally no filler materials or spacers should be used along the charge strings in the bore-hole. The explosive is fluid or viscous, in contrast to pulverulent or granular, and is coherent in the sense that the fluid or viscous phase is continuous around any solids present and the explosive cohesive both when pumped and in string form. The explosive is pumpable, i.e. move as a single phase under pressure and have a sufficiently low viscosity to be moved through the charging hose, possibly with liquid lubrication, under not too high pressure loss. The explosive may be pumpable at elevated temperatures but it is preferred that it can be pumped at ambient temperatures. Explosives termed "repumpables" may be used. The explosive may be sensitized by microspheres or by mecanical or chemical gassing or any combination therebetween. Microsphere sensiti=ed explosives may be affected by pumping but are volume stable in the string after pumping. Gassed explosives offers the possibility to after-foam in the bore-hole following extrusion, either by pressure release or continued chemical reaction, the latter to be preferred, e.g for the purpose of increasing sensitivity or further reduce explosive strength in relation to the pumped explosive. The additional foaming may with preference take the explosive to lower than pumpable densities. Independent of the sensitizing method the pumped explosive should be regarded as the bulk form of the explosive for the purposes of the invention. The preferred explosive types are gel explosives, slurry explosives and in particular water-in-oil type emulsion explosives, all optionally with additional solid oxidizer salts in amounts not destroying the cohesive character of the explosive. All these explosives are extensivly described in the patent literature. The emulsion explosives, having a continuous fuel phase and a discontinuous oxidizer phase, should preferably have a substantially all-oil fuel phase in order to be readily pumpable. The emulsion should have density reduced in relation to the void-free matrix of at least 10 percent by weight of the matrix, preferably at least 15 percent. In absolute terms the density could be below 1.3 g/cc and preferaby below 1.25 g/cc. The lower limit is highly flexible and dependent on the degree of strength reduction desired. For high energy explosives or microsphere sensitized explosives the density reduction is generally limited to 40 and preferably also above 30 percent or in absolute terms above 0.8 or above 0.9 g/cc. Gassed and after-foamed emulsions may have even lower densities, with density reductions of at least 50 and even 60 percent or absolute densities down to 0.7 g/cc or even down to 0.5 g/cc. A suitable apparatus for carrying out the method of the invention and for charging explosive in a controlled volume amount per bore-hole length unit should include a vessel for the explosive and a charging hose for insertion into the bore-hole and a conduit connecting these devices. The conduit should incllude a pump able to feed the pumpable explosive at a controlled and stable volume rate, which rate should preferably be variable in order to allow different degrees of partial charging. Positive displacement pumps giving small flow rate variations, such as "monopumps", may be used. In case the explosive is to be chemically gassed the conduit may include an inlet for gassing agent, normally a liquid, and possibly a vessel for such an agent and a pump for moving and dosing the agent into the conduit. A mixing device should be present in the conduit after the inlet in order to evenly distribute the agent in the explosive. The pump may act as a mixing device but it is preferred to arrange the inlet after the pump and insert a mixer after the inlet, preferably a static mixer. In the extreme, the mixer may be positioned at the end of the charging hose, optionally with a small tube paralell with the hose to an inlet immediately prior to the mixer. In order to reduce the pressure requirements in pumping the explosive it is suitable to arrange for introduction of a lubricating fluid between the conduit and hose interior surface and the explosive. The fluid may be water but is preferably an aqueous solution of oxidizing salts similar to those present in the explosive itself. The arrangements may comprise an inlet for the lubricating liquid ending in an annular chamber surrounding the channel of the conduit and having a ring opening towards the channel for forming a liquid ring around the centrally fed explosive. The apparatus should include means for moving the hose. At least these means should allow forward movement of the hose when inserted into the bore-hole and driving menas for withdrawing the hose at controlled rate. The rate can be variable during charging operation but is preferably constant. The rate is preferably adjustable. Suitably the driving means also assists in the forward motion of the hose. Any type of moving means fulfilling these requirements can be used for the purposes of the invention. One type of such moving means include opposed wheels or bands gripping a part of the hose therebetween and driving means connected to at least one of the opposed wheels or bands able to move the hose at least in the withdrawal direction. A preferred device of this kind is described in the Swedish patent 8903101-7 (465 566). The device is highly flexible and allows strongly variable feeding speeds both in forwand and reverse directions. Another preferred type of hose moving means include a winder or reel with guiding means for receiving turns of the charging hose on its peripheral parts, preferably in a monolayer, and driving means for rotating the winder in a direction withdrawing the hose from the bore-hole towards the winder at a controlled rate. This device may include disengaging means allowing manual unwinding of the hose under rotation of the winder. The guiding means may include restricting means preventing radial expansion of hose turns on the winder, except at a point of unwinding, whereby the hose is securely retained on the winder and pushing actions are also made possible. The apparatus should also include adjusting means for setting the ratio between the controlled pumping rate and the controlled hose withdrawal rate, in order to expel the explosive in the volume rate desired to give the string characteristics stated. The adjusting means may include means for varying the pumping rate and/or the withdrawal rate. A simple, yet for many purposes sufficient, arrangement is to use adjusting means giving constant withdrawal rate and variable pump rates. Hydraulic motors are preferred driving means for pump and withdrawl means, allowing a broad range of stable rates. Summary of drawings Figure 1 illustrates a simplified bore-hole pattern of an underground tunnel with different bore-hole types. Figure 2 illustrates the formation of an explosive string in a bore-hole according to the invention. Figure 3 illustrates schematicly a preferred apparatus for string formation according to the invention. Description of drawings The tunnel profile of Figure 1 shows a number of bore-holes provided in the rock face 1. Several contour holes 2 along roof and side walls are suitably weakly charged with for example a partial charging degree of 25 percent as defined. Holes next to the contour holes (not shown) are charged to an intermediate degree of for example 50 percent. Remaining holes, including drift holes 3 and foot holes 4 as well as holes 5 close to the central empty cut 6 can be entirely filled to a charge degree of 100 percent. The same explosive is suitably used for all the holes. Figure 2 shows in side view a bore-hole 21 in rock 22. Through charging hose 23 is pumped an explosive under simultaneous withdrawal of the hose. A uniform string of the explosive is formed which string only partially fills up the available radial space in the hole. Figure 3 shows in perspective view a suitable charging apparatus for the method of the invention. The apparatus comprises a vessel 31 containing a pumpable explosive 32 feeding into a pump 33 with motor 34. A vessel 35 containing gassing agent 36 is via inlet 37 connected to the conduit, generally designated 38. A static mixer 39 is provided to mix the gassing agent with the explosive. A vessel 40 containing lubricating liquid 41 is connected to an annular chamber 42 surrounding the central part of conduit 38. The chamber 42 has a ring opening 43 through which the liquid feeds into the conduit between the inner surface thereof and the centrally pumped explosive. The conduit 38 terminates in the central part of a winder or reel 44. A charging hose 45, connected to the central termination of conduit 38, is placed in a monolayer of turns 46 on the periphery of inner cage 47. The inner cage is rotatable at constant speed by actuating means 48. An outer cage 49 is rotatable coaxially with, but independent of, inner cage 47 and have periphery means limiting radial movements of charging hose turns 46. At exit 50 the hose can be withdrawn or extended under simultaneous winding or unwinding on rotating inner cage 47. Example 1 A water-in-oil type emulsion explosive was prepared by forming a fuel phase containing 7 parts by weight of a process oil (Nyflex 8130) including 1 part emulsifier (Lubrizol 5691B) and 93 parts oxidizer phase, containing 66 percent by weight ammonium nitrate, 18 percent sodium nitrate and 16 percent water. The two phases were emulisfied at about 75 centigrades high shear mixer (CR-mixer) to a final viscosity of about 37.000 cps at the preparation temperature. To this matrix glass microspheres (Q-cell 723) were added in an amount sufficient to give a warm emulsion density of about 1.18 g/cc corresponding to a cold emulsion density of about 1.20 g/cc. This emulsion was charged into a various steel tubes having outer diameters between 20 and 51 mm and wall thicknesses of about 3 mm. When completely filled with the emulsion, and initiated with detonator and 50 g primer, the charges detonated with velocities between 5048 and 5652 m/sec. An estimated velocity for an unconfined charge of 50 mm diameter is about 5000 m/sec. The same type of emulsion was charged into two 40 mm steel tubes of the same wall thickness and a length of 3 m in an amount corespondig to half the cross-section area of the tube. The detonation velocity was measured at 7 points separated 30 cm along the tube. Apart from the first measuring sections, where detonation velocity was affected by the primer used, the detonation velocity stabilized at between 2000 to 2500 m/sec. Example 2 Transparent plastic tubes of inner diameter 42 mm were partially filled with explosive according to Example 1, using an apparatus similar to that described in relation to Figure 3, although without the parts relating to gassing. The liquid ring was fed with water in an amount of 3 percent by weight of the emulsion flow. The apparatus had hydralic motors for the winder and the pump with adjustable hose and pump rates. A great number of charging tests were done with the apparatus, in each case with different although during charging constant pump and winder rates. Strings obtained were examined and weighed. The strings had small size variations and expected and reproducable results were obtained with various apparatus settings. Example 3 In a commercial tunnel drifting one of the contour holes were charged according to the invention and initiated together with the other holes in the round. The charged hole was about 41 mm in diameter and had a length of 3.7 m and was initiated from the bottom with a 29 x 200 mm NG (Dynamex) primer. The hole was charged with the same type of emulsion as in Example 1 in an amount of 0.3 litre per meter of the hole, corresponding to a partial filling degree of about 23 percent of the cross-section area. The detonation velocity was measured over two distances in the bore-hole, well separated from the initial part affected by the primer. The velocity was measured in such single bore-holes of a round at six different occations. The velocities measured varied between 1320 and 2420 m/sec and no detonation interruptions were experienced. The charge operated in the intended way, leaving readily visible semicircular bore-hole remnants on the rock face. Example 4 In the same tunnel as in Example 3 all the bore-holes of the round (exept some control holes) were charged with the same type of explosive and the same apparatus. All the holes were completely filled with the explosive, except the contour holes for the walls and the roof, which were partially filled to 23 percent, and the holes inmediately inside the contour holes which were partially filled to about 50 percent. The control holes in the contour were charged with conventional plasstic 22 and 17 mm tube charges containing granular explosives (Gurit). The round gave good advance and fragmentation. The contour was undamages with equivalent good results for holes shot with emulsion and tube charges. Example 5 About 70 full tunnel profiles has been charged and shot substantially as in Example 4. Under slightly varying conditions similar results were obtained with the same chaging pattern. With fully charged holes next to the profile the final rock face was damaged. Example 6 An emulsion matrix according to Example 1 is prepared. No microspheres are added but the oxidizer phase contains an acidic acid additive in an amount of 0.2 percent by weight of the entire emulsion. Using the apparatus of Figure 3, a gassing agent containing 35 percent aqueous solution of sodium nitrite and an accelerator of natrrium thiocyanate is fed from the gassing agent vessel into the conduit in an amount sufficient to give a density of about 1.15 g/cc after extrusion and a reaction time of about 20 minutes, which density then remains substantially constant. The same tunnel profile as in Example 4 is charged with the explosive with roughly the same weight amount of explosive per meter bore-hole in corresponding types of holes over the profile. The filled up holes are charged to an initial filling degree of about 85 to 90 percent, allowing space for radial expansion during foaming. The contour holes and the holes immediately inside the contour holes are only partially filled after gassing as in the previous example, although with a density somewhat lower of about 1.0 g/cc which is obtained by a slightly higher ratio of gassing agent to matrix when charging these holes. Similar results are obtained as with the rounds using microsphere sensitised explosive.

Claims: A method for charging explosives in substantially horizontal bore-holes, with a loading density reduced in relation to that corresponding to the complete fill up of the bore-hole diameter and with the explosive in bulk form, wherein a charging hose with an end opening is introduced into at least one substantially horizontal bore-hole of a blasting round, characterized by pumping an explosive in fluid or viscous form as a cohesive mass through the charging hose at a controlled rate into said substantially horizontal bore-hole(s), the charging hose being withdrawn at a controlled rate simultaneously with said pumping, and the pumping and withdrawal rates being adjusted so as to form while exiting from said hose end opening a coherent string of said cohesive explosive mass with said exiting string only partially filling up the substantially horizontal bore-hole diameter. A method according to claim 1, characterized by partially filling up said bore-hole diameter such that the cross-section area of the exiting string is between 10 and 90 percent, preferably between 20 and 80 percent, of the bore-hole cross-section area over a substantial part of bore-hole length. A method according to claim 1 or claim 2, characterized in that the pumping and withdrawal rates are adjusted so as to give a varying string cross-section area over bore-hole length. A method according to claim 3, characterized in that the string cross-section area decreases towards the bore-hole opening. A method according to claim 1 or claim 2, characterized in that the pumping and withdrawal rates are adjusted so as to give a substantially constant string cross-section area over a substantial part of the bore-hole length. A method according to any one of the preceding claims, characterized in that hose withdrawal rate is maintained substantially constant. A method according to any one of the preceding claims, characterized in that igniting means are introduced into the bore-hole. A method according to claim 7, characterized in that the igniting means are positioned close to the innermost part of the bore-hole and that the pumping and withdrawal rates are adjusted so as to give an explosive amount at the igniting means in excess of the string amount in the main part of the bore-hole length. A method according to claim 8, characterized in that said excess amount is obtained by a delay of hose withdrawal after pumping start. A method according any one of the preceding claims, characterized in that the fluid or viscous explosive is selected from the group consisting of slurry explosives, water-in-oil emulsion explosives and mixtures thereof with solid oxidizer salts. A method according to any one of the preceding claims, characterized in that the fluid or viscous explosive contains microspheres as a sensitizing agent. A method according to any one of the preceding claims, characterized in that the fluid or viscous explosive contains a gassing agent as a sensitizing agent. A method according to claim 12, characterized in that the gassing agent in the exiting string is further reacted in the bore-hole to radially expand the string by foaming. A method according to claim 13, characterized in that after radial expansion the string substantially fills up the bore-hole cross-section. A method according any one of the preceding claims, characterized in that two or several different bore-holes of the blasting round are charged to different ratios of string cross-section area to bore-hole cross-section area. A method according to claim 15, characterized in that at least one bore-hole is charged with a string filling up the bore-hole cross-section area. A method according to any one of the preceding claims, characterized in that the bore-hole has a diameter of between 25 and 150 mm (1 and 6 inches), preferably between 36 and 100 mm (1.5 and 4 inches). A method according to any one of the preceding claims, characterized in that the string cross-section area is between 1 and 20 cm 2 , preferably between 2 and 15 cm 2 . A method according to any one of the preceding claims, characterized in that the velocity of detonation in the string is between 500 and 3500 m/sec, preferably between 1000 and 2500 m/sec.

Inventors: ENGSBRATEN BJOERN, MAGNUSSON ROLF, ENGSBRATEN, BJOERN, MAGNUSSON, ROLF, Engsbraten, Björn
Applicants: NITRO NOBEL AB
Application year: 1994
Main classification: F42D1
IPC codes: F42D1, E21D9